Semiconductor devices are generally fabricated from a wafer formed of a silicon substrate having an overlying, light transmissive thin film layer, such as SiO.sub.2. During device fabrication, a coating of photoresist is applied over the light transmissive layer and then the photoresist is selectively exposed to light radiation either by contact or projection printing techniques. The unexposed portions of the photoresist are then removed, to leave a mask pattern on the light transmissive layer. The portions of the light transmissive layer not covered by the photoresist mask are then etched away to expose the silicon substrate therebeneath to form diffusion or contact windows to permit the creation of the desired device features. Etching may be accomplished by the use of acids or a reactive plasma.
In practice, etching of the wafer is usually monitored by a human operator. A technique presently used to allow manual determination of the depth and rate of etching to facilitate monitoring includes the step of directing a laser beam towards an area of the light transmissive layer not covered by the coating of photoresist. The light transmissive layer typically has an index of refraction between 1 and 2 so that the beam of light incident thereon is partially reflected at the surface thereof and is partially transmitted therethrough for reflection from the silicon substrate therebeneath. Because of the finite thickness of the light transmissive layer, there is a path length difference between the reflected beams, causing the beams to interfere.
During etching of the light transmissive layer, this path length difference changes, causing the composite intensity of the reflected, interfering beams to exhibit periodic maxima and minima. A detector detects the composite intensity of the beams and produces an output signal which varies accordingly. The detector output signal, which approximates a sinusoid during etching, is recorded by a strip chart recorder. The operator counts the number of cycles in the recorded sinusoid representing the variation over time of the beam intensity and manually computes the etch depth in accordance with this count. The period of the recorded sinusoid is then measured, and the frequency thereof is manually calculated. After the frequency has been calculated, the etch rate can be computed.
There are several disadvantages associated with manual determination of the etching process parameters in this fashion. Operator measurement of the number of cycles and the period of the sinusoid recorded by the strip chart recorder and subsequent computation of the depth and rate of etching, may result in error. Further, when many wafers are separately undergoing processing at the same time, as often occurs at many semiconductor manufacturing facilities, the time spent by the operator to individually calculate the etching parameters associated with each wafer may delay his or her performance of other tasks. For instance, while the operator is busy making the measurements and performing the calculations for one wafer, he or she may not be able to intercede during the etching of another wafer in a timely fashion in order to make whatever adjustments are required before too much material has been removed.
Accordingly, there is a need for a method for automatically measuring the semiconductor etching process parameters.